Field of the Invention
This invention relates to a treated fine powder of silica constituting a toner for developing an electrostatic latent image in a process of image forming, such as electrophotography, electrostatic recording, electrostatic printing or the like, and to an electrostatic image development toner having such a treated fine powder of silica.
Hitherto, various electrophotography methods are known, including those disclosed in U.S. Pat. No. 2,297,691, and in Japanese Patent Publication Nos. 42-23910 and 43-24748.
Generally, development methods applicable to these electrophotography methods ar grouped into dry development methods and wet development methods. The former are further grouped into methods using a two-component type developer and methods using one-component type developer.
Conventionally, in these dry development methods, a fine powder formed by dispersing a dye and/or a pigment in a natural resin or a synthetic resin is used. For example, a fine powder having a particle size of 1 to 30 .mu.m, which has been and formed by pulverizing a binder such as polystyrene having a colorant disposed therein is typically used as a toner. As magnetic toners, powders containing particles of a magnetic material such as magnetite are used. In the case of a method using a two-component type developer, a toner is ordinarily used by being mixed with carrier particles, such as glass beads or iron particles.
It is necessary for each type of toner to have positive or negative charge according to the polarity of an electrostatic latent image to be developed. For this effect, a compound called a charge control agent is ordinarily added to the toner.
Various chemical substances are added to a toner according to image fixation performance and other characteristics required.
In particular, a method of adding a fine silica powder to the outer surface of a toner particle to achieve a desired fluidity of the toner is widely used for the purpose of improving image characteristics such as resolution, density uniformity and fog level.
This method addresses the problem of the increased degree of dependence of these image qualities upon the environment. The use of a toner prepared in this manner is presently enabled by a special means, e.g., a heater provided in a copier or other additives. However, remodeling a main unit of a copier to improve image characteristics also results in an increase in price. Further the addition of other additives may cause different problems. This problem is particularly serious in the case of a full-color copier which must be capable reproducing a half-tone image with high fidelity. In this technical field, therefore, there is a strong need to develop a fine silica powder which, when added to the outer surface of a toner, can significantly reduce changes in the performance of the toner under environmental influences.